1. Field of the Invention
The present invention relates to a power supply method and a power supply device for a portable telephone such as a cellular phone (or cordless telephone), and more particularly to a power supply method and a power supply device for a portable telephone having measures against voltage drop making it difficult to transmit and receive.
2. Description of the Related Art
The telephone number memory of a portable telephone can not be referred to if the battery is used up. When going out, the telephone number memory of the portable telephone is often an only telephone directory for the user of the portable telephone. If the communication function of the portable telephone does not work, there is other communication means such as pay phone, but if the portable telephone is the only telephone directory, there is no other means of knowing the telephone number, and the user cannot make a call.
If the battery is spent or the battery voltage level is lowered, it is inevitable that communication cannot be made with the portable telephone, but it is desired to be continuously allowed to refer to the telephone number memory.
A related prior art disclosed in Japanese Laid-Open Patent Application (JP-A-Heisei, 5-244733) teaches xe2x80x9cPOWER SOURCE CHANGEOVER DEVICE FOR PORTABLE TELEPHONExe2x80x9d. In this power source changeover device, usually, the changeover means is feeding the supply voltage of the main battery to the logic circuit and communication circuit. When the output voltage of the main battery drops, a voltage drop signal is issued from the voltage monitoring means and the changeover means changes over the supply of power source from the main battery to the sub battery. When changed over to the sub battery, power supply to the communication circuit is cut off by the power cut-off means. As a result, even if the voltage of the main battery drops, only the logic function can be used.
Japanese Laid-Open Patent Application (JP-A-Heisei, 9-172402) discloses xe2x80x9cBATTERY SAVING DEVICE FOR A MOBILE WIRELESS APPLIANCExe2x80x9d. In this battery saving device, the power source is supplied from the battery to the telephone set circuit through a switch circuit unit, and the remaining level of the battery is detected by a remaining level detector, and when the remaining level reaches 5% (equivalent to 5-minute talk), the controller turns off the switch in the switch circuit unit, and the power source of the battery is not supplied into the telephone set circuit. The manual switch is usually set in ON position. Therefore the battery remainder equivalent to 5-minute talk can be reserved, and it avoids the case of impossibility of use due to complete discharge of battery if attempted to use portable telephone.
Japanese Laid-Open Patent Application (JP-A-Heisei, 9-139981) discloses xe2x80x9cPORTABLE TELEPHONE AND A PORTABLE IMAGE COMMUNICATION TERMINAL DEVICExe2x80x9d. The device comprises an EEPROM which stores the caller""s telephone number when talking and maintains the stored caller""s telephone number if the communication is interrupted due to battery consumption in the portable telephone, and a controller which reads out the telephone number from the EEPROM and displays on the screen of a display unit when the power source is recovered by replacing the battery if the communication is interrupted due to battery consumption in the portable telephone.
Japanese Laid-Open Patent Application (JP-A-Heisei, 10-289038) discloses xe2x80x9cPORTABLE INFORMATION TERMINAL DEVICExe2x80x9d. In this device, a constant voltage circuit issues plural reference voltages. A comparator compares the plural reference voltages and the supply voltage of the battery, and issues a comparison signal corresponding to the supply voltage of the battery. A controller, according to the comparison signal, controls the supply voltage supplied to the transmitting and receiving circuit and voice talk circuit, and the supply voltage supplied to the video processing circuit, video display unit and back-light by controlling with the power source changeover switch. It hence prevents sudden interruption of communication when the battery remaining level decreases.
Japanese Patent No. 2775105 discloses xe2x80x9cLIQUID CRYSTAL DISPLAY DEVICExe2x80x9d. In this liquid crystal display device, when driving a back-light liquid crystal display unit by battery, drop of battery voltage is detected and displayed by alarm. More specifically, it comprises the back-light, a back-light driving circuit, battery voltage detecting means, drive controlling means for issuing a control signal to the driving circuit depending on the result of detection by this detecting means, and issuing a control signal for stopping the driving circuit when the detecting means detects that the battery voltage is lower than a predetermined value, and invalidating means for invalidating the control signal of the drive controlling means for a predetermined time when the power source is turned on and operating the driving circuit.
It is hence an object of the present invention to present a power supply method and a power supply device for a portable telephone capable of referring always to at least the telephone number memory only.
Means for achieving the object is expressed as follows. Technical matters appearing in the expression are indicated by numerals and symbols enclosed in parentheses xe2x80x9c( )xe2x80x9d. The numerals and symbols coincide with the technical matters for composing at least one embodiment or example out of plural embodiments and examples of the present invention, in particular the reference numerals and reference symbols given to technical matters expressed in the drawing corresponding to the embodiment or example. Such reference numerals and reference symbols clarify the correspondence or relation between the technical matters in the claims and the technical matters in the embodiment or example. Such correspondence or relation does not mean that the technical matters in the claims are interpreted as being limited in the technical matters in the embodiment or example.
The power supply method for a portable telephone according to the present invention comprises the steps of detecting the remaining level of a battery (9), and stopping power supply to other circuits corresponding to a first group of circuits except necessary circuits (5, 7) corresponding to a second group of circuits that need electric power for reading telephone number from a telephone number memory (13) and displaying the read out telephone number when the remaining level is lower than a specified value. The other circuit does not mean all circuits except the necessary circuits that needs electric power. The other circuits are preferred not to include circuits consuming a very small amount of power for a very short time for operating indispensable parts in case of emergency such as the back-light and the display circuit as described later. Only the telephone directory function is remaining, and it avoids a worst case of impossibility of any operation due to drop of battery level and failure of function of portable telephone.
The specified value is usually a voltage level at which it is difficult to transmit and receive, in other word, the portable telephone can not carry out a transmitting and a receiving operation, and substantially it is the telephone function losing voltage. The necessary circuits include the telephone number memory (13) and a display unit (7) to display a telephone number. It is further preferred to include a display for notifying that it is impossible to transmit and receive but possible to display a telephone number.
When the remaining level is lower than the specified value, it is preferred to change over or switch the power source from a battery (main battery) to a spare battery (11). Provision of the spare battery (11) securely avoids an occurrence of such a case that the function of the telephone directory becomes inoperative. In this case, the specified value may be a voltage level at which the portable telephone can not carry out a transmitting and a receiving operation and a calling operation of a telephone number, so that the electric power in the main battery can be used up without fear. When the remaining level becomes lower than the specified value, it is preferred not to drive a back-light (22).
The power supply device for a portable telephone according to the present invention comprises a telephone number memory (13), a battery (9), a telephone controller (5), and a battery remaining level detector (10) which is connected to the telephone controller (5) to monitor the output voltage V of the battery for always detecting the remaining level of the battery (9). The telephone controller (5) includes a controller for stopping power supply to other circuits (2, 3, 4) corresponding to a first group of circuits except necessary circuits corresponding to a second group of circuits that need electric power for reading a telephone number from the telephone number memory (13) and displaying the read out telephone number when the battery remaining level drops below a specified value.
Further, the power supply device includes a backlight (22), the telephone controller (5) is also preferred to include a controller for not driving the back-light (22) when the battery remaining level drops below the specified value. Moreover, the power supply device includes an emergency switch (15) for driving the back-light in case of emergency, it is preferred to drive the back-light (22) by the emergency switch (15) even after the telephone controller (5) has controlled not to drive the back-light (22).